South Korea and the United States launched a major joint air force exercise on Friday, as US Defence Secretary Ashton Carter visits Seoul.
The two-week-long "Max Thunder" drill will involve around 100 military aircraft, including jet fighters and bombers carrying out tactical bombing runs, the South Korean Air force was quoted as saying in various media reports.
Around 1,400 servicemen from both countries will be taking part in the drill which is held twice a year and comes against the backdrop of the annual Key Resolve and Foal Eagle joint exercises.
As Carter arrived in Japan on the first leg of his two-nation tour of the key US military allies in Asia, the North fired two surface-to-air missiles off its west coast as a means of expressing its displeasure.
Carter, who arrived in South Korea on Thursday, said the missile firings were a reminder of "how dangerous things are" on the Korean peninsula and how important it was to strengthen the US-South Korea alliance.
The United States has close to 30,000 troops permanently stationed in South Korea and would assume operational command of both armed forces in the event of a conflict with the North.
The US is also pushing to deploy a ballistic missile defense system in South Korea as a deterrent to military provocation by the North.